// 2.3.1 ���������ε����
#include <iostream>
#include <cmath>
using namespace std;

int main()
{
	double a, b, c, s, A;
	cout<<"输入:"<<endl;
	cout<<"a,b,c=";
	cin>>a>>b>>c;
	s = (1.0 / 2.0)*(a+b+c);
	A = sqrt(s*(s-a)*(s-b)*(s-c));
	cout<<":"<<A<<endl;
	
	return 0;
	
} 
